1. Field of the Invention
This invention relates to a process for the production of finely divided, urea group-containing solid polyisocyanates by reacting urea group-free organic polyisocyanates with water, the organic polyisocyanate initially being converted in a large excess of water (based on the quantity of polyisocyanate) according to the invention in the presence of a protective colloid and optionally an emulsifier, to a stable emulsion or suspension and the reaction of the polyisocyanate with water optionally being carried out by addition of catalysts and/or bases, to produce urea group-containing polyisocyanates.
2. Description of the Prior Art
Processes for the production of urea group-containing polyisocyanates are known in the prior art. The production of reactive resin compositions by reacting an aromatic diisocyanate with from 0.1 to 0.9 mol of water per mol of diisocyanate in a solvent, which is capable of dissolving both the water and the diisocyanate and which shows a basic reaction in the presence of water, for example pyridine, is described for example in U.S. Pat. No. 2,597,025. The resin compositions are stable in storage with the exclusion of moisture and harden in the presence of atmospheric moisture within a very short time to produce an infusible, insoluble material, presumably a polyurea. The urea group-containing polyisocyanates dissolved in the reaction medium are not isolated. The production and isolation of urea group-containing polyisocyanates of defined composition is not an object.
U.S. Pat. No. 2,757,184 describes the production of substituted di(isocyanatophenyl)-ureas by reacting about 1 mol of water with 2 mol of diisocyanate in inert solvents containing oxygen atoms in the molecule, such as ethers, esters and ketones. In some solvents of this group, for example diethylethers or more preferably diisopropylethers (see DE-Applicattion No. P 3,419,429, Example 4) products are obtained which correspond practically in their NCO-content to the theoretical value. This production process, however, suffers from the disadvantage that organic solvents must be used, which pose considerable safety problems due to their volatility and oxidation tendency (ethers) and thus give rise to high processing costs. The products produced in these solvents, which occur in the form of small needle shaped crystals, likewise have a tendency to agglomerate during the work-up (filtration), so that an expensive grinding process is subsequently necessary, if a finely-divided product is to be obtained. If the product is ground, for example, in an air jet mill, a very finely-divided powder is obtained which has a very low bulk density (about 0.10-0.15 g/cm.sup.3) and thus demands a large storage volume.
U.S. Pat. No. 3,906,019 describes the production of di(isocyanatotolyl)ureas, wherein one of the reaction constituents, water or toluylene-diisocyanate, is added in excess and acts as the reaction medium. The resulting di(isocyanatotolyl)ureas are insoluble in both reaction constituents and immediately crystallize out after their formation. The reaction between water and diisocyanate is catalyzed by addition of compounds of the Lewis-base-type or the Lewis-acid-type. Water is preferably added in stoichiometric excess and then acts as reaction medium.
From 5 to 15 parts of water are preferably added to from 20 to 40 parts of diisocyanate. The products have an NCO-content, which is reduced by about .gtoreq.10 rel % in comparison to the theoretical NCO-content. 60, 83 and 65% by weight of urea diisocyanate are given as the yield of the three described examples. If the examples, in which water is used in excess, are adjusted (column 3, lines 1-16; column 3, line 45; column 4, line 7) several important disadvantages of this process can be established.
1. The reaction between the diisocyanate and the water takes place very exothermically with the given water and catalyst quantities; the reaction temperature in the described small-scale mixture on this page, lines 8-20, can only be maintained under 40.degree. C. by cooling with ice sodium chloride bath as described. This fact is likely to lead to great difficulties, particularly in largescale technical usage, since a considerable quantity of CO.sub.2 is released within a short time.
2. The resulting urea diisocyanate agglomerates immediately and precipitates on the wall of the reaction vessel in the form of a thick, hard layer. After reaction, this can only be removed from the reaction container with difficulty, for example by cooling the reaction vessel including the product to -60.degree. C., whereby the product which has become brittle and fragile can then be removed from the wall of the container in great chunks. After drying, a grinding process is necessary in all cases.
3. The pyridine, preferably used as a catalyst during reaction, is not completely removed by the work-up described in the patent (washing with hexane or ethyl acetate). An extremely unpleasant and aggravating pyridine odor thereby clings to the products, which can also cause problems during further use of the products through evaporation and uncontrollable catalysis.
4. The yields are all under 85%, so that ecological problems occur during the removal of the separated liquid phase.
The production process described in U.S. Pat. No. 3,906,019, which is limited to toluylene-diisocyanate, in which an excess of water is used for the first time as reaction medium suffers in the disclosed form from substantial disadvantages. An object of the present invention was to develop a process for the production of solid urea group-containing polyisocyanates in water as the reaction medium, in which these disadvantages no longer arise. It was attempted, in particular, to produce the polyisocyanate particles in as large a yield as possible and in such a finely-divided form, that the products can be worked-up problem-free and a subsequent grinding process becomes unnecessary for most uses.